Sing a different tune
Definition 1 of 2
Pro Tip 1/3Opinion Reversal
Used when someone completely changes their stance on a topic, often shockingly so. 
My brother used to dislike pop music, but he's singing a different tune after the concert.
Pro Tip 2/3Critical to Positive
Often used when someone goes from criticizing to praising something. 
After seeing my grades, my parents are singing a different tune about my studying habits.
Pro Tip 3/3Persuasive Influence
Highlights the impact of new evidence or persuasion on one's previous beliefs. 
Once I presented the data, they started to sing a different tune and supported the research.